Method and apparatus for imaging with multiple exposure heads

ABSTRACT

An imaging system is equipped with two or more exposure heads that are each able to image either a separate media sheet or a portion of a single media sheet loaded on a media carrier. The imaging time for any combination of media sheets is minimized by providing for the adjustment of the spacing between the dual exposure heads whenever the media configuration changes. In imaging a unitary image using two exposure heads to each image a sub-image, any discontinuity between the end of the first sub-image and the start of the next sub-image is reduced by changing the traversing speed of one of the exposure heads by a fractional amount.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of application Ser. No. 10/688,901, filed Oct.21, 2003 now U.S. Pat. No. 7,256,811.

FIELD OF THE INVENTION

The invention relates to imaging systems and more particularly toimaging systems which form an image on a recording media using multipleexposure heads.

BACKGROUND OF THE INVENTION

Imaging systems that are capable of imaging films, lithographic plates,flexographic plates, proofing materials and other media types are wellknown in the art. In the printing industry, laser based exposure headsare commonly used to form an image on a lithographic plate forsubsequent use in a printing operation on a printing press. Some imagingsystems are capable of printing on multiple media formats such asplates, films, and proofing media.

A common imaging system architecture provides an exposure head whichgenerates one or more modulated beams or channels and an imaging mediacarrier for securing a media sheet. The beams are scanned over the mediaby a scanning means which produces relative motion between the mediasheet and the beams. The scanning means may comprise, for example, anexternal drum, internal drum, or a flatbed scanning system. In anexternal drum system the media is held on a rotatable drum and the beamsfrom the exposure head are scanned over the media surface by acombination of drum rotation and translation of the exposure head.

A common problem in the design of imaging systems is providingsufficient imaging speed to meet the media preparation requirements ofthe industry. Particularly in the printing industry, where a largecapital investment in printing press equipment dictates that pressesshould be kept running at high duty cycles, the time taken to prepare aplate for press may be a limiting factor in the printer's overallworkflow.

U.S. Pat. No. 5,887,525 to Okamura et al. describes a machine forsimultaneously making two printing plates for newspaper printing. Themachine has two exposure sections in series to speed up the productionof plates for a newspaper press. In U.S. Pat. No. 5,795,689 to Okamuraet al. the speed of a machine for making newspaper printing plates isincreased by using two exposure heads in parallel to scan differentareas of a plate, thus reducing the time taken to prepare a plate foruse on the press. The exposure heads may each write images that areduplicates or the image written by each exposure head may be different.

U.S. Pat. No. 5,934,195 to Rinke et al. describes a flatbed system thatis capable of simultaneously exposing two separate single-wide plates,each having the same or a different image thereon, or a singledouble-wide plate, each half of which has the same or a different imagethereon.

There remains a need for better methods and apparatus for imaging withmultiple exposure heads.

SUMMARY OF THE INVENTION

A first aspect of the invention provides an imaging apparatus comprisinga media carrier and at least two exposure heads. Each exposure head isdisposed to image a portion of a single sheet of media secured on themedia carrier, or one of at least two sheets of media secured on themedia carrier. An adjustable spacer is provided for moving the exposureheads relative to each other to change the spacing therebetween.

In another aspect of the present invention a method of imaging with atleast two exposure heads is provided. The method comprises loading atleast one sheet of media on a media carrier and adjusting the spacingbetween the exposure heads in accordance with the number and size ofmedia loaded on the media carrier. A portion of a single sheet of mediasecured on the media carrier, or one of at least two sheets of mediasecured on the media carrier are then imaged by each exposure head.

In yet another aspect of the invention a method for aligning twoexposure heads for imaging a unitary image on a media is provided. Theunitary image is partitioned into two sub-images. The method comprisesimaging a first test image with one of the exposure heads and imaging asecond test image with the other exposure head, the second test imageadjoining the first test image. The degree of misalignment between theexposure heads is determined by examining the adjoining portion betweenthe test images. The traversing speed of at least one of the exposureheads is adjusted in accordance with the determined degree ofmisalignment.

For an understanding of the invention, reference will now be made by wayof example to a following detailed description in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In drawings which illustrate by way of example only preferredembodiments of the invention:

FIG. 1 is a perspective view of an imaging system for imaging twoseparate media sheets;

FIG. 2 is a perspective view of a system for imaging a single largemedia sheet;

FIG. 3 is a perspective view of a pair of exposure heads on a commonleadscrew;

FIG. 4 is a perspective view of a pair of exposure heads each on anindependent leadscrew;

FIGS. 5-A to 5-D are views of various aligning systems;

FIG. 6 is a process flowchart depicting a method of the presentinvention;

FIG. 7 is a schematic view of an imaging media and the relativepositioning of the exposure heads;

FIGS. 8-A to 8-C are a series magnified views of a portion of theimaging media shown in FIG. 7;

FIG. 9 is a schematic diagram showing a test image for aligning twoexposure heads; and

FIG. 10 is a simulated moiré pattern illustrating one specific alignmentmethod according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an imaging engine 10 having a drum 12. Drum 12 is rotatableabout a central axis 14. Two sheets of media 16 and 18 are secured ondrum 12. A first exposure head 20 is disposed to image media sheet 16and a second exposure head 22 is disposed to image media sheet 18.Exposure heads 20 and 22 are each attached to a corresponding carriage24. Carriages 24 are traversed by rotating leadscrew 26, thus drivingleadscrew nuts 28, which are attached to carriages 24.

Exposure heads 20 and 22 are independent imaging units, each responsiveto separate data and control signals, but traversed by a commonleadscrew 26. Leadscrew rotational drive is provided by a suitable motor(not shown) such as a stepper motor or a servo motor. The position ofthe exposure heads 20 and 22 along the length of the leadscrew 26 may bedetermined by keeping a count of the number of steps applied to thestepper motor in relation to a previously established home position.Alternatively, other well-known linear or rotary mechanisms and linearor rotary encoding techniques may be employed to translate and keeptrack of the lateral position of an exposure head.

Imaging engine 10 is capable of imaging a media in two different modes.In a first imaging mode shown in FIG. 1 exposure head 20 images mediasheet 16 and exposure head 22 images media sheet 18. The images may bedifferent or identical. In a second imaging mode shown in FIG. 2 each ofthe exposure heads 20 and 22 image a portion of a single large mediasheet 40 loaded on drum 12. This reduces the imaging time over thatwhich would be required if the media sheet 40 were to be imaged by asingle exposure head.

In an alternative third mode of operation, the two or more exposureheads may be used to provide some redundancy. In the event of a failureof one of the exposure heads the imaging can be done by the otherexposure head until the failed exposure head is replaced or repaired.The imaging time in this mode will be slower, but this represents auseful system reliability feature to a user who is severely impacted bydowntime.

In practice, there are some problems associated with the simpleembodiments shown in FIG. 1 and FIG. 2 in that the spacing between theexposure heads 20 and 22 is fixed by leadscrew 26. For a specific headspacing imaging speed is only maximized when imaging a media that has awidth approximately twice the spacing between heads (for one large sizedsheet or two smaller sheets, each smaller sheet approximately half thesize of the large sheet). Any other sizes of media sheet will generallyhave less-than-optimal imaging times.

Another problem occurs when imaging a unitary image on a single largemedia sheet, with each exposure head imaging a portion of the unitaryimage. The line along which the two image portions join (the “stitchline”) will generally show some discontinuity unless the two exposureheads are very precisely spaced. The spacing may drift with time andenvironmental conditions making it necessary to periodically re-spacethe heads.

In an embodiment of the invention shown in FIG. 3, exposure heads 20 and22 are traversed on a common leadscrew 26. Exposure head 22 has a fixedleadscrew nut 50 while exposure head 20 has a rotatable leadscrew nut52. Nut 52 is connected to exposure head 20 via a bearing (not shown)allowing nut 52 to rotate freely while simultaneously preventing anyrelative longitudinal motion between the nut 52 and exposure head 20.Exposure head 20 is additionally equipped with an auxiliary drive motor54 which may be a stepper motor. Auxiliary motor 54 provides rotationaldrive to nut 52 via a pulley 56 driving a belt 58. The spacing betweenheads 20 and 22 can be adjusted by rotating nut 52 in response tocontrol signals from a motor controller 59.

The adjustment will generally be made before or after an imagingoperation and may advantageously be executed during a retrace cyclewhile the exposure heads are returning to a home position on completionof an image. The adjustment is preferably performed automatically inresponse to a control signal from controller 59 but this is not mandatedand some of the benefits of the invention may be realized in a manuallyadjusted system.

As may be appreciated by a person skilled in the art, many othermechanisms for driving nut 52 may be employed to effect the adjustment.Other mechanisms for adjusting the spacing between exposure heads 20 and22 may also be provided instead of, or in addition to, a nut 52 which isrotatable relative to its carriage 24. For example, the adjustmentbetween nut 52 and exposure head 20 may be provided by a separatetranslation stage, such as a secondary leadscrew or other lineartranslation stage employed to move exposure head 20 relative to nut 52.In the embodiment shown in FIG. 3, the main traversing drive is stillprovided by leadscrew 26. Alternatively it is also possible to providerotational drive to both nuts 50 and 52, while holding leadscrew 26stationary. In another alternative embodiment shown in FIG. 4, a pair ofexposure heads 60 and 62 are each independently driven by leadscrews 64and 66. The drive to leadscrews 64 and 66 is provided by separate motors(not shown). The separate motors may nevertheless be synchronized tooperate from a common system synchronization clock (which may also beused to control drum rotation). Conveniently, stepper motors may be usedfor the leadscrew drive since they allow both precise stepping andcontrol, but any other suitable type of motor or motor/encodercombination may also be used.

In another alternative embodiment, only one of the exposure heads isdriven by the leadscrew. The other exposure head is coupled to the firstexposure head via a coupling to space them apart. The second head movesin tandem with the first. The spacing between heads is adjusted byvarying the length of the coupling. In one embodiment the couplingcomprises a bar having a length which can be varied by heating orcooling the coupling bar to thermally expand or contract it. The spacingis accurately maintained by controlling the temperature of the bar. Aheater controlled by a controller (not shown) may be used to control thelength of the coupling bar. Thermal adjustment provides very finecontrol of the spacing and a fixed bar provides a rigid connectionbetween exposure heads removing any leadscrew effects from the spacing.

In a method according to the invention the imaging operation comprisesthe following steps:

-   -   (a) determining the format of the loaded media—e.g. a single        media sheet or a pair of media sheets;    -   (b) adjusting the spacing between the exposure heads to        correspond to either half the width of the single media sheet or        to align with the spacing between a pair of media sheets,        depending upon which format is present; and,    -   (c) imaging either the single media sheet, with each exposure        head imaging approximately half of the media sheet, or a pair of        media sheets with each exposure head imaging one of the sheets.        If there are more than two exposure heads then, for printing on        a single media sheet, the exposure heads may be space apart by        1/0 times the width of the media sheet, where 0 is the number of        exposure heads.

Advantageously, by adjusting the spacing between two exposure heads inaccordance with the size of the media being imaged, the overall imagingtime is reduced for any combination of media.

In the case where two or more separate media are imaged, each with aseparate exposure head, the adjustment between the exposure heads neednot be particularly precise. Many imaging systems have edge detectionhardware for detecting the edge of the media sheet, optically orotherwise. One common optical edge detection method senses thediscontinuity in surface reflectivity between the media and the drumsurface. A precision of roughly 5:m can be achieved, which is quiteadequate for most printing. In FIG. 5-A, media sheets 16 and 18 aresecured on drum 12. Each of exposure heads 20 and 22 are equipped withan edge detection beam 72. The exposure heads 20 and 22 are traversedover the edges 90 and 92, and the edge locations recorded. The imagingdata may then be arranged such that each image is correctly located onthe media 16 and 18.

In the case where each exposure head images a portion of a single mediasheet the spacing between exposure heads should be more preciselyadjusted to avoid a visible discontinuity between the joined imageportions. Simple edge detection may not be sufficiently accurate forother than low resolution imaging. It has been found that even errors ofaround ⅕th of a pixel may be discernable on some sensitive media. At2400 dpi this translates into a sensitivity of around 2:m, which is analmost impossible accuracy to hold through mechanical tolerancing alone.A practical approach is to periodically align each exposure head to atarget located on the drum. FIG. 5-D shows a drum 12 with a target 70positioned at a fixed location on the surface of drum 12. The targetprovides a common alignment point for each of the exposure heads 20 and22. Exposure head 22 is shown with an auxiliary beam 72 impinging ontarget 70. By first aligning exposure head 22 to target 70, and thenmoving exposure head 20 to align with target 70, the spacing between theexposure heads may be determined and adjusted. Alternatively, theimaging heads could be aligned to separate targets, spaced a knowndistance apart, albeit with potentially lower accuracy.

One specific embodiment of the target is shown in FIG. 5-B. Beam 80 fromexposure head 20 is directed towards lens 82. Lens 82 is recessed intothe surface of drum 12. The light gathered by lens 82 is directed to aposition sensitive detector (PSD) 86 via mirror 84. PSD 86 generates asignal 88 responsive to the position of a beam 90 on the sensitive areaof the PSD 86 and is able to indicate movement of the beam 90 in thedirection of arrow 92. Lens 82 magnifies the displacement to increasethe sensitivity of the target thus amplifying the motion 92 occurring atthe surface of PSD 86.

In an alternative embodiment shown in FIG. 5-C, a target 94 hasnon-reflective areas 96 and reflective areas 97. Reflective areas 97 arelocated in the shape of a “Y” (on its side). The geometry of thereflective target 94 and specifically the angle between the “Y” branchesis accurately determined prior to installing the target. A suitabletarget 94 may be constructed from a thin sheet of stainless steel usinga lithography and chemical etching process to pattern the “Y” shape,guaranteeing a precise, known, geometry. Alternately the target may beseparately characterized using well known measuring techniques.

In operation an auxiliary laser beam from the imaging head 20 or 22 isscanned over target 94 along line 98, the laser beam traversing twobranches of the “Y” in succession. The reflection of the laser beam fromthe target 94 is monitored by a light sensor such as a photodiode (notshown) that converts the light intensity reaching the light sensor intoan electrical signal. As each reflective branch of the “Y” target istraversed, the light sensor signal changes sharply defining a transitionfrom non-reflective area 96 to reflective area 97 and back again tonon-reflective area 96. The signal from the light sensor representingthis transition is used to precisely determine the location of the Ybranch.

Advantageously target 94 allows both X and Y co-ordinates of the laserbeam to be simultaneously determined in a single traversing of thetarget along direction 98. The Y co-ordinate is determined as the halfway point between the encoder readings at the two signal transitions.The X co-ordinate is

determined from the following formula:

$X^{\prime} = {\frac{d}{2} \cdot {\tan\left( \frac{\theta}{2} \right)}}$where X′ is the X displacement of the beam (at line 98) fromintersection point 99 of the two branches of the “Y”, d is the distancebetween the signal transitions, and □ is the angle between the branchesof the Y. For □=90E the tan term equates to 1 and X′=d/2.

Alternatively, the target 94 may be viewed by a video camera. Theresulting image is analyzed using pattern matching software (systemsthat include a video camera and pattern matching software are available,for example from Cognex Corp, USA).

Advantageously, it is not necessary for the beam to traverse thereflector target at any specific location, as long is it traverses bothbranches of the “Y”. The third branch of the “Y”, is used in as aconvenient Y co-ordinate determination when there is no need for an Xco-ordinate determination. It should be readily appreciated that thetarget may also be constructed from two angled reflective lines, notnecessarily intersecting and not necessarily oriented as shown.

In some high resolution imaging systems, a discontinuity may still occurat the join between the two sub-images of a unitary image imaged on asingle media sheet, even when the exposure heads are precisely spaced.For the best results it may be more practical to do a final fineadjustment based on inspection of a test pattern imaged on the media. Inthe embodiment shown in FIG. 2 the beam (or beams) from exposure heads20 and 22 are scanned over the media 40 by simultaneously rotating drum12 while translating exposure heads 20 and 22, each exposure head thuscircumscribing a helical pattern around the drum. The discontinuity maybe caused by a simple displacement between the end of one sub-image andthe start of the other sub-image, or it may be caused by slightdifferences between the imaging beams that write the image in theadjoining area. A discontinuity is more likely to be apparent whenimaging at high resolution. Another factor that influences theappearance of the discontinuity is the media. Some media are more likelyto reveal or accentuate imaging artifacts than others.

FIG. 6 is a flowchart depicting of a method for imaging a unitary imageon a media with two exposure heads. Data defining a unitary image isreceived in step 140. In step 142, the data is partitioned to define twosub images 144 a and 144 b. The unitary image data file is split intotwo independent files, each containing a sub-image 144 a or 144 b. Eachof the sub-images 144 a and 144 b are sent to a corresponding exposurehead in step 146 a and 146 b. Preferably, each exposure head will imageapproximately 50% of the image but this is not mandated.

In step 148 the sub-images are imaged on a single media sheet to form aunitary image on the media. It should be evident that the goal is thatthere should be no easily discernable difference between an imagewritten by two or more exposure heads and an image writtenconventionally by a single exposure head.

FIG. 7 depicts an imaging media 40 that has been imaged by exposureheads 20 and 22. As previously described the scanning action may producea series of slanted helical bands 110 across the imaging media. Eachband may be a few mm in width or more and is imaged by a number ofparallel independent beams or channels. It is well known in the art tore-arrange the data transmitted to the exposure head to ensure that,while the imaging bands may be tilted by some angle to the edge 116 ofthe media 40, the actual image imparted is orthogonal to the imagingmedia.

The first exposure head 20 starts imaging sub-image 124 at band 112. Thesecond exposure head 22 starts imaging sub-image 126 at band 122. If itis required to image right to the edge 116 of the imaging media 40, band112 may also span across the edge 116 of imaging media 40. The last fullwidth band imaged by the exposure head 20 is band 118. The sub-images124 and 126 are divided along line 100 according to the previouslydescribed partition point in the unitary image. Line 100 may be called astitch line or a stitch. Since line 100 may not have been chosen exactlyat the end of full band 118, exposure head 20 may be required to imagepartial band 120 in order to complete the first sub-image. When partialband 120 is being imaged by exposure head 20, band 122 has already beenimaged by exposure head 22. The partial band 120 must be preciselyaligned with band 122 to avoid the appearance of a discontinuity at theboundary therebetween.

In order to align the end of partial band 120 with the beginning of band122 it is necessary to calculate how many individual beam widths are inthe first sub-image 124, and then arrange for exposure head 20 to plot apre-determined number of full bands, followed by a partial band with thelast imaged beam being close to, but not necessarily overlapping, thebeginning of band 122. Since the minimum width that can be imaged is anindividual beam width, the alignment will generally be in error by lessthan one individual beam width. Unfortunately, at higher resolutions andfor some imaging media types an error of a single beam width or less maybe clearly apparent as a discontinuity in the resulting image.

This effect is further explained with reference to FIGS. 8-A to 8-C,which are magnified views of region 8 indicated in FIG. 7. In FIG. 8-Athe last full band 118 and the partial band 120 of sub-image 124 areshown, as is the first band 122 of the second sub-image 126. The end ofband 118 joins partial band 120 along line 119. Lines 130 do not definethe bands but rather define the extents of individual imaging beams 132.Each band comprises a plurality of such individual imaging beams 132.The gap indicated at 134, which is smaller than the width of individualbeam 132, results from not imaging an individual beam over the gap 134.If an individual beam were written in gap 134 it would also overlap thebeginning of band 122. This situation is depicted in FIG. 8-B where thebeginning of band 122 has been overwritten. This is shown schematicallyas a dark line 136, which results from the double exposure. In the casesshown in FIG. 8-A and FIG. 8-B the discontinuity may be discernable. Thesize of the gap 134 or the overwritten portion 136 can always bearranged to be less than the width of one individual imaging beam, sinceif the gap were more than this width it would be a simple matter towrite one more individual beam to reduce the width of gap 134. In thisway, the misalignment may be always restricted to an individual beamwidth or less.

This remaining misalignment cannot be easily corrected since imagingoccurs on a pixel-by-pixel basis, the pixel being the minimumaddressable element defined by an individual imaging beam 132. Returningto FIG. 7 it should readily be appreciated that the spacing betweenadjacent bands 110 is determined by the speed of translation of exposureheads 20 and 22. This is usually adjusted so that no separation betweenthe bands is evident when the individual beams are correctly spaced forthe chosen imaging resolution. The ability of an imaging engine toproduce such a geometrically accurate image is important, particularlyin the printing industry, where color separations must be accuratelyregistered to print properly. The required registration accuracy mayvary for different printing presses and printing resolutions. At 1200dpi an accuracy of around 30:m is generally sufficient. At thisresolution individual beams having widths of approximately 20:m aretypically used. By fractionally increasing or decreasing the speed oftranslation of exposure heads 20 and 22, the gap 134 or overwrittenportion 136 shown in FIGS. 8-A and 8-B may be effectively eliminated.The fractional increase in speed need only account for the width of anindividual beam or less. Consequently, the effect on the geometricaccuracy of the final image is negligible. The spacing between adjacentbands is affected by only a very small amount.

As an example, considering a 22-inch wide plate where each of thesub-images are 11 inches wide, at 1200 dpi there would be approximately13,200 individual beam widths in each sub image. For an exposure headwith 240 parallel channels, this corresponds to 55 bands. The maximumcorrection required for eliminating the gap or dark band at the stitchis 20:m (one individual beam width or less). This corresponds to anadjustment of 0.36:m at each band or a speed change of ∀ 0.007%, whichis undiscernible from band-to-band but corrects for the discontinuity atthe adjoining area.

In practice, the actual speed change required may be determinedempirically by writing a number of images on one or more imaging mediasheets, each with successive small changes in speed of translation. Thespeed that produces the least visible discontinuity is chosen for use insubsequent imaging operations. Advantageously, as shown in FIG. 9, asingle sheet of imaging media 40 may be imaged with a test setcomprising a plurality of test strips 150 made at different speeds oftranslation. The test strip 152 with the least visible discontinuitynear line 100 indicates the optimal translation speed. This process hasthe added advantage that if there is some difference between the beamsproduced by exposure heads 20 and 22, this difference may at the sametime be at least partially corrected by the choice of the visually bestimage in the test set shown in FIG. 9.

Another method for determining the required speed change is todeliberately overwrite a set of vertical lines from each exposure head.The resulting moiré interference pattern may be examined to determinethe required speed change. This method is explained with reference toFIG. 10, which shows a first set of lines 160 imaged at a small angle toa second set of lines 162. In the depicted example the lines are imagedusing an imaging system of the type that effects a helical scanning ofthe drum. The first set of lines 160 are imaged by the first exposurehead and the second set of lines 162 by the second exposure head. Thesmall angle between the lines may be introduced by disabling a number ofchannels on one of the exposure heads. This changes the helix angle forthat head as the traversing speed is automatically increased by thesystem to compensate for having fewer imaging channels. The two sets oflines 160 and 162, offset at a small angle to each other, will produce amoiré pattern as shown in FIG. 10. For the situation where the pitchbetween the lines and the angle between the sets of lines is known, asis the case here, the position of the dark band or fringe 166 isindicative of the misalignment between the two patterns and thus theoffset between the two exposure heads. Advantageously a scale 168 may beimaged alongside the sets of lines so that the spacing can be directlyread off the imaging media at the location of fringe 166. Alternatively,the position of the light fringe 164 may be used to calculate theoffset. As will be readily apparent to a person of skill in the art theuse of lines to generate moiré patterns is convenient but not mandated.Any repetitive feature will create a moiré pattern that is usable forthe purposes of the method described. For example, a plurality of dotsin a regular grid when overlapped with another plurality of dots on aregular grid will also produce a moiré pattern.

Once accomplished, the adjustment may be susceptible to drift due tochanges in the environmental temperature. Many imaging systems use asteel leadscrew for advancing the exposure heads, the steel having anexpansion coefficient of around 12 ppm/EC. For a 500 mm distance betweenexposure heads the leadscrew will thus expand or contract by ˜12:m forevery 2EC change in temperature. Such a minor change in environmentalconditions would have the effect of completely negating the alignment.The change may be accommodated by precisely measuring the temperature ofthe leadscrew and adjusting the scanning speed to compensate for anychanges. The temperature measurement may need to take account oftemperature gradients in the imaging system and will possibly requiretwo or more temperature measurements at different points along theleadscrew. Alternatively, the expansion of the leadscrew with respect tothe frame supporting the engine may be measured directly using ameasuring device such as a Linear Variable Displacement Transducer(LVDT). The drum, leadscrew and carriage ways are typically all held ina frame, which may be of a different material than the leadscrew. It isparticularly important to measure the difference between the expansionof the frame and the leadscrew. Thus an LVDT or the like attached to theframe at the floating end of the leadscrew and contacting the endthereof is ideally disposed to measure the quantity of interest.

Another factor that may affect the alignment is the pointing stabilityof the imaging beams produced by the exposure heads. The pointingdirection is typically a property of the optical systems used to formthe imaging beams. In some instances, it may be necessary to provideintermittent or continuous monitoring and adjustment of the beampointing to ensure that the image-to-image alignment is maintained for areasonable time.

Yet another factor that may need to be taken into account is the overallscaling of the images. Many imaging systems are carefully adjusted toprovide accurately scaled images by imaging and measuring test images onan XY measuring table or the like. Scaling factors are calculated andapplied to the imaging system as a machine calibration. It should beunderstood that such a calibration and the alignment of the images willgenerally be interrelated and will need to be performed together so thatimages are aligned and appropriately scaled.

There may also be a requirement to duplicate a portion of the data inthe region of the partition point to deliberately overlap the images atthe partition point. This need arises in the imaging of some types ofmedia wherein adjacent bands are commonly overlapped by one or more beamwidths. This feature is particularly useful for some types of thermallysensitive imaging media where subsequent exposures are not additive.Overlapping has been found to even out the boundaries between adjacentbands. Overlapping may also be useful in aligning sub-images produced bydifferent exposure heads. The overlapped data is a repeat of thepreviously written data and writing may occur at full beam power or atreduced beam power. The duplication of the data is preferably taken intoaccount when partitioning the image file into sub-images.

Although the foregoing discussion has been focused on a specificembodiment of an imaging engine the method may be applied to a widerange of imaging architectures where it is desired to write a singleimage with two or more exposure heads. Where the exposure heads share acommon translation means the same translation speed change is applied toboth exposure heads equally, thus limiting the correction to beingperformed with two such exposure heads. However, where the exposureheads are independently translated the invention may be applied tosystems having two or more exposure heads. Similarly the method is alsoapplicable to the situation where the distance between exposure heads isnot adjustable, in which case the size of the imaging media and/or thesize of the image to be written will determine the proportion of theimage to be written by each exposure head.

The method is also applicable to other imaging architectures such asinternal drum systems and flatbed systems. In such cases, while thescanning may be different the requirement still exists to stitchtogether two or more sub-images and as such, the translation speed maybe altered in the manner described to reduce the appearance of thediscontinuity.

The data partitioning may be achieved in a variety of different waysdepending on the data format and the configuration of the system. Forexample instead of splitting the image into two separate sub-imagefiles, a pointer may be used to indicate the point of partition betweenthe two sub-images. It should also be understood that other formattingsteps may follow the partitioning step.

It should also be noted that other methods of scanning beams across animaging media are well known. One example of an alternative scanningmethod is to image a circumferential band while the exposure head isheld stationary, whereafter the exposure head is indexed to a newposition to image the next circumferential band. During the indexingoperation the imaging ceases until the exposure head is in position toimage another circumferential band, lined up alongside the previous one.While the invention has been described in relation to a helical scanningsystem, it is also applicable to other scanning methods employed in theindustry.

As will be apparent to those skilled in the art in light of theforegoing disclosure, many alterations and modifications are possible inthe practice of this invention without departing from the spirit orscope thereof.

1. An imaging apparatus comprising: a media carrier; at least twoexposure heads spaced apart from one another, each exposure headdisposed to image a portion of a single sheet of media secured on themedia carrier; and an adjustable mechanism for moving the exposure headsat different traverse speeds relative to each other while the exposureheads we concurrently imaging the corresponding portions of the media.2. The imaging apparatus of claim 1 wherein the exposure heads aredisposed to image the media secured on the media carrier by directingimaging beams towards the media.
 3. The imaging apparatus of claim 1wherein the adjustable mechanism moves the exposure heads relative toeach other to change a spacing therebetween while the exposure head aredirecting imaging beams towards the corresponding portions of the singesheet of media secured on the media carrier.
 4. An imaging apparatuscomprising: a media carrier; a plurality of exposure heads spaced apartfrom one another, each exposure head disposed to image correspondingportions of media supported on the media carrier; a mechanism for movingthe exposure beads; and a controller for adjusting a traverse speed ofat least one exposure head to change a spacing between the exposureheads while the exposure heads are concurrently imaging thecorresponding portions of the media supported on the media carrier. 5.The imaging apparatus of claim 4 wherein the each exposure head isdisposed to image a portion of a single sheet of media supported on themedia carrier.
 6. The imaging apparatus of claim 4 wherein each exposurehead is disposed to image one of at least two sheets of media supportedon the media carrier.
 7. The imaging apparatus of claim 4 wherein theexposure heads are disposed to image the corresponding portions of themedia supported on the media carrier by directing imaging beams towardsthe media.
 8. The imaging apparatus of claim 4 wherein the mechanismmoves the exposure heads along a traversing direction, and thecontroller adjusts the traverse speed of the at least one exposure headto change the spacing between the exposure heads along the traversingdirection.
 9. The imaging apparatus of claim 4 wherein the mechanismmoves the at least one exposure head along a traversing direction andthe media carrier moves relatively to the at least one exposure headalong a direction that is not parallel to the traversing direction. 10.The imaging apparatus of claim 9 wherein the controller causes the atleast one exposure head to move along the traversing directionconcurrently as the media carrier moves relatively to the at least oneexposure head.
 11. The imaging apparatus of claim 9 wherein the at leastone exposure head images one or more bands on the media, the one or morebands extending in a skewed orientation with respect to both thetraversing direction and the direction that is not parallel to thetraversing direction.
 12. The imaging apparatus of claim 4 wherein themechanism moves the at least one exposure head along a traversingdirection and the media carrier moves relatively to the at least oneexposure head along a direction that is transverse to the traversingdirection.
 13. The imaging apparatus of claim 4 wherein the mechanismcomprises one or more leadscrews, and the media carrier movestransversely to an axis of the one or more lead screws.
 14. The imagingapparatus of claim 4 wherein the mechanism moves the at least oneexposure head along a traversing direction and the at least one exposurehead images one or more bands on the media, the one or more bandsextending in a direction that is skewed with respect to the traversingdirection.
 15. The imaging apparatus of claim 4 wherein the media issecured to the media support.
 16. The imaging apparatus of claim 4wherein the mechanism moves the at least one exposure head along atraversing direction and the media moves relatively to the at least oneexposure head along a direction that is not parallel to the traversingdirection.
 17. The imaging apparatus of claim 16 wherein the controllercauses the at least one of the exposure heads to move along thetraversing direction concurrently as the media moves relatively to theat least one exposure head.
 18. The imaging apparatus of claim 4 whereinthe media carrier comprises a cylindrical drum.
 19. The imagingapparatus of claim 4 comprising a flatbed imaging system.
 20. An imagingapparatus comprising: a media carrier; a plurality of exposure headsspaced apart from one another along a traversing direction, at least oneexposure head disposed to direct imaging beams to form bands on mediasupported on the media carrier, wherein the bands extend in a directionthat is not parallel to the traversing direction; and an adjustablemechanism for moving the exposure heads relative to each other to changea spacing therebetween while each exposure head is concurrently moving.21. The imaging apparatus of claim 20 wherein the each exposure head isdisposed to image a portion of a single sheet of media supported on themedia carrier.
 22. The imaging apparatus of claim 20 wherein eachexposure head is disposed to image one of at least two sheets of mediasupported on the media carrier.
 23. The imaging apparatus of claim 20wherein the adjustable mechanism changes the spacing between theexposure heads along the traversing direction.
 24. The imaging apparatusof claim 20 wherein the media carrier moves relatively to the at leastone exposure head along a path that is not parallel to the traversingdirection.
 25. The imaging apparatus of claim 20 wherein the media movesrelatively to the at least one exposure head along a path that is notparallel to the traversing direction.
 26. The imaging apparatus of claim20 wherein the media is secured to the media carrier.
 27. The imagingapparatus of claim 20 wherein the adjustable mechanism changes thespacing between the exposure heads while each exposure head isconcurrently imaging corresponding portions of the media supported onthe media carrier.
 28. A method for imaging comprising: supporting amedia on a media carrier; imaging the media with a plurality of exposureheads spaced apart from one another, wherein each exposure head isdisposed to image corresponding portions of the media supported on themedia carrier; and adjusting a traverse speed of at least one exposurehead to change a spacing between the exposure heads while the exposureheads are concurrently imaging the corresponding portions of the mediasupported on the media carrier.
 29. The method of claim 28 comprisingmoving the exposure heads along a traversing direction while theexposure heads concurrently image the corresponding portions of themedia supported on the media carrier, wherein the traverse speed of theat least one exposure head is adjusted to change a spacing between theexposure heads along the traversing direction.
 30. The method of claim29 comprising moving the media carrier relative to at least one exposurehead while the at least one exposure head is moved along the traversingdirection, wherein the media carrier moves relative to at least oneexposure head in a direction that is not parallel to the traversingdirection.
 31. The method of claim 28 comprising moving the mediacarrier relative to at least one exposure head while the at least oneexposure head is directing imaging beams towards the media.
 32. Themethod of claim 28 comprising moving the media relative to at least oneexposure head while the at least one exposure head is directing imagingbeams towards the media.
 33. An imaging apparatus comprising: a mediacarrier; at least two exposure heads spaced apart from one another, theexposure heads comprising a plurality of channels disposed to emit beamsto image media secured on the media carrier; a mechanism for moving theexposure heads relative to each other to change a spacing therebetweenwhile each exposure head is concurrently imaging corresponding portionsof the media secured on the media carrier; and a speed controllerconnected to the mechanism for moving to allow a traverse speed of atleast one of the exposure heads to be controlled sufficiently preciselyto adjust a position of a last channel to within less than one beamwidth.